Satellite communication systems provide a means by which data, including audio, video and various other sorts of data, can be communicated from a transmitter at one location to a receiver at another location. Satellite communication systems are currently being used on mobile platforms, such as civilian airlines and privately owned aircraft to provide entertainment and internet access to the passengers. Military platforms, such as aircraft and ships, currently use satellite communication systems to receive and transmit various types of information, including strategic and tactical information.
Satellite communication systems require an antenna to receive signals from, and transmit signals to, a satellite. The antenna typically must be pointed accurately at the satellite. A satellite antenna positioner is typically used to point the antenna at the satellite. It is common for these antenna positioners to have two axes of motion (e.g., elevation and azimuth). In the case of a system mounted on an aircraft, the elevation and azimuth that will point the antenna to the satellite can be calculated if the following information is known: (1) the location and attitude of the aircraft; and (2) the location of the satellite, assuming the relative alignment of the antenna to the body of the aircraft is known. In most commercial airliners and military aircraft, the attitude of an aircraft is determined by a position and attitude measuring device (PAMD), such as an inertial reference unit (IRU).
Such systems typically provide the attitude of the aircraft in terms of three orthogonal axes: roll, pitch, and yaw. Errors in alignment of the antenna with respect to the PAMD will cause pointing errors (i.e., the antenna will not be pointed accurately at the desired satellite when using information from the PAMD to calculate the parameters, such as azimuth and elevation, for pointing the antenna). These alignment errors can be defined as roll, pitch and yaw errors. The antenna can be “peaked” to correct for these errors for a particular orientation. Peaking involves finding the antenna direction that results in the greatest signal strength received from the satellite through the antenna. These corrections are determined within the antenna positioner. Accordingly, such corrections will be determined in the two axes of elevation and azimuth used by the antenna positioner.
While the corrections can be converted from azimuth and elevation to a three dimensional Cartesian coordinate system, a problem exists in that such corrections will only be accurate for that particular orientation of the mobile platform. Applying these corrections to the azimuth and elevation calculated for other orientations will not accurately point the antenna. In fact, applying such corrections may result in even greater pointing errors in some orientations.
It can be seen that accurately aligning the antenna to the PAMD of an aircraft is important when using the PAMD output to position a satellite antenna. However, performing the alignment poses challenges. Therefore, there is currently a need for a simple and accurate means by which to align a satellite antenna to a mobile platform, such as an aircraft frame or PAMD within an aircraft.